The invention relates to a filtration system comprising a device for the ultrasonic cleaning of its filtering elements and a corresponding cleaning process.
Systems are known which are used for the filtration of liquids and which comprise, within a casing, a set of filtering elements made of porous material(s) or membranes. The use of membranes is a widely employed, versatile, and effective separation process. Different membrane types, ranging from micro-filtration (MF) to reverse osmosis (RO), are capable of removing suspended particles and microorganisms to rejecting virtually all ions in solution, respectively.
During the operation of the filtration system, impurities from the fluid progressively fill the interstices of the porous material and/or form a cake layer on top of the porous material. In order to maintain satisfactory conditions of operation of the filtration system, it is necessary to regenerate the filtering elements after a certain operating time.
A first method of regeneration consists in replacing all the filtering elements when the loss of head through these elements has exceeded a specified limiting value, for a given throughput. This method involves disadvantages, because it necessitates the demounting of a part of the system and because it involves the handling of a large volume of waste materials, comprising all the impurities retained by the cleaned filtering elements and these filtering elements themselves. This leads to increased labor cost and an increased period of non-availability of the filtration system; furthermore, it may be necessary to handle and to process large quantities of waste materials after cleaning of the filter. Additional difficulties arise when the filtration systems of the type described hereinabove are utilized for the processing of fluids containing impurities consisting of dangerous materials. Requiring the replacement of such filtering elements increases the risk of exposing maintenance personal to such contaminates.
A second process for the regeneration of the filtering elements of a filtration system consists in isolating the system from its normal circuit and, without demounting it, causing a certain quantity of clean fluid to pass in countercurrent through the filtering elements. These elements are then cleared of the particles retained during the operation of the system. This backwashing method avoids the demounting of the filter and the replacement of the filtering elements. However, backwashing is not ideal in that the volume of fluid necessary to carry out the cleaning may be fairly large causing possible filtration membrane degradation, and/or may necessitate the utilization of costly processing stations for the resulting highly contaminated effluent. Backwashing also requires system shutdown while the filter is cleaned.
A third process for the regeneration of the filtering elements of a filtration system consists using chemical cleaning to remove particles from fouled filtration membranes. Currently, practices of chemical cleaning are mostly based on recommendations from membrane manufactures. Some of them supply proprietary cleaners while others use commercial chemicals. Accordingly, in addition to having to isolate the filtration system from the filtration circuit, membrane degradation may be problematic due to the use of inappropriate chemicals. Further disadvantages include chemical costs, waste handling and disposal costs, and the need for additional operator safety and training to use this type of regeneration process.
A fourth process for the regeneration of the filtering elements of a filtration system consists in emitting ultrasonic energy through a casing or pressure vessel to remove particles from fouled filtration membranes. The emission of ultrasonic energy within the casing creates an acoustic pressure field. If the amplitude of this pressure field is sufficient, cavitational bubbles generate shock waves, as well as the turbulent micro-currents resulting from cavitational collapse permits the extraction of the particles filtered and collected by the porous medium. However, using ultrasound has not been without its disadvantages. For example, prior art methods have used a membrane housing or other barrier between the ultrasonic energy and the membrane surface, thereby requiring additional power input. Additionally, because such systems have an intermediate medium through which to transmit ultrasonic energy, such prior systems have also be unable to provide a uniform distribution of the ultrasonic energy to the fouled membrane surface.
Accordingly, there is a continued desire in the industry for a filtering system and corresponding ultrasonic cleaning operation that does not necessitate the demounting of the filter or disconnecting the filter from the operating circuit, which provides a more uniform distribution of ultrasonic energy, and which has a lessened power requirement.